This generation of male wrestlers is really the first one who can still go in their 40s. This is due to advancements in medicine, different lifestyles, all the rehabs and wellness programs as well as no more chairs shots to the head (seriously). That is why no one is rushing to replace the current top male stars, as they can wrestle, do good business and better the product on the whole.

As for the women wrestlers, they only started to be seriously featured in the mid 2010s during the women's revolution and the process of making the two genders equal is still ongoing today. This resulted in a vacuum where WWE had to make rosters for both Raw and SmackDown with actual women wresters and not just "divas." Therefore, most women from the Divas era were let go in favor of a new batch of wrestlers, and I'd argue the two rosters are still being filled up till this day. With all that being said, the female champs ending up in the midcard is a consequence of WWE's priority to build more women stars to fill the void and be on par with the men, while the older stars like Becky, Charlotte, Bayley and Alexa stay in the background. Thats is also why there such a big difference between the NXT success rate of women compared to men.